Temperature stable ultrasonic delay line composed of sio2 and mgf2 and method of making same



. X5? 3 9 536 a 5 O 6 F r o 1970 u. u. I-KASER 3,536,506

TEMPERATURE STABLE ULTRASONIC DELAY LINE COMPOSED OF 3102 AND M F AND METHOD OF MAKING SAME '1. Filed Aug. 14, 1968 l r Y INVE/VTOR 0. B. FRASER w BY 4% E ATTORNEY United States Patent- US. Cl. 106-58 2 Claims ABSTRACT OF THE DISCLOSURE The specification describes a delay line having low temperature sensitivity. It comprises a heterogeneous particulate body of SiO and MgF Since SiO has a negative temperature coefiicient of delay and MgF a positive c0- efiicient of delay the materials are mixed in proportions resulting in a zero temperature coefficient.

This invention relates to ultrasonic delay lines which are relatively insensitive to temperature changes.

Excessive variation of delay time with temperature has been a principal consideration in the construction of delay lines. The most satisfactory solution to date takes advantage of certain zero temperature coefiicient glasses which are found to be effective delay media and are relatively insensitive to normal changes in temperature. However, the glass compositions which exhibit this ability have been found to have chemical instabilities. These instabili ties, though small, are suflicient to significantly impair the delicate delay time-temperature relationship.

Consequently, there continues to be a need for an inexpensive material from which delay lines can be easily fabricated and which exhibits a low dependence of delay time with temperature.

This invention is directed to such a delay media and borrows somewhat from the concepts contained in application Ser. No. 601,716, filed Dec. 14, 1966 by J. T. Krause. In that application a zero temperature coefficient material was prepared by joining together two discrete bodies of dilferent acoustic materials, one material having a positive temperature coefiicient, the other a negative coefi'icient. The relative lengths of the two bodies can be selected so that the composite structure exhibits a zero temperature coefiicient. Whereas this solution is acceptable, the presence of the boundary between the media is a source of loss and unwanted reflection. According to this invention the problem is overcome by mixing together silicon dioxide and magnesium fluoride in finely particulate form and hot-pressing the mixture into a compact body. Silicon dioxide has an appreciable negative temperature coeflicient of delay and magnesium fluoride has a large positive coefiicient of delay. When mixed in the proper proportions a composite mixture having a zero temperature coeflicient of delay can be produced. These particular compounds do not react appreciably during the hot-pressing operation so that the resulting body is a true physical mixture. It is apparent that for the purpose of the invention each component of the mixture must retain its physical properties.

A body of particulate matter would normally be expected to be very lossy but a compacted body of these Patented Oct. 27, 1970 particular components exhibits surprisingly efiicient ultrav sonic transmission.

These and other aspects of the invention may be more fully appreciated from a consideration of the following 5 detailed description. In the drawing:

The figure is a perspective view of a compacted particulate delay line made in accordance with the invention. The novel ultrasonic medium can be described in terms of an ultrasonic delay line, a typical embodiment of which is shown in the figure. The delay medium 10 is composed of a compacted body of SiO and MgF The ends of the delay medium are coated with metal electrodes 11 and 12 and conventional transducers such as quartz or barium titanate, 13 and 14, are affixed to the electrodes. Metal 1 films 15 and 16 cover the exposed face of the transducer; 17, 18 refer to the input terminals and 19, 20 designate the output terminals.

The composite body 10 was made by the following procedure. Particulate SiO and MgF were thoroughly 20 mixed and then pressed at 980 C. and 20,000 p.s.i. for fifteen to twenty minutes. The particle size was 2a. The appropriate particle sizewill be somewhat dependent on the frequency of operation anticipated for the device but generally would average less than 5 Samples of different SiO /MgF weight ratios were measured for temperature dependence of delay time. A composition having a ratio of 0.306 yielded a delay coeflicient of +14 p.p.m./ C. and a composition having a ratio of 0.419 yielded a delay coeflicient of 13 p.p.m./ C. These ratios are considered to describe the useful range of low temperature coefiicient materials. The optimum ratio for a Zero TC material lies in the range of 0.35 to 0.38 (accounting for experimental error and other variables). The internal friction of these mixtures, at 1.5 mHz., is comparable and in some cases lower, than that in typical isopaustic glasses. Aging tests show no measurable room temperature aging over an eight week period after heating to 110 C.

What is claimed is:

1. An ultrasonic delay line having low temperature sensitivity comprising a delay medium consisting essen tially of a dense heterogeneous mass of SiO and MgF mixed together in a weight ratio of Si0 to MgF falling within the range 0.35 to 0.38.

2. A method for making a delay medium having low temperature sensitivity comprising the steps of:

mixing together SiO and MgF in finely particulate form, the particles being less than 5 1. in size, and the weight ratio of SiO to MgF being in the range 0.35 to 0.38, and

compacting the particulate mixture at a pressure of at least 20,000 p.s.i. and a temperature of the order of 980 C.

References Cited PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 

